This invention relates to methods and arrangements for reproducing audio signals in which a pitch control device increases or decreases the amplification of the audio signals in one or more defined frequency ranges of the audio signals and a volume control device increases or decreases the amplification of the audio signals in a full effective frequency range of the audio signals.
In such arrangements the pitch control device reduces the amplification in the defined frequency ranges if the amplification of the audio signals in the full effective frequency range is increased by the volume control device and increases the amplification of the audio signals in the defined frequency ranges if the volume control device decreases the amplification of the audio signals in the full effective frequency range. In corresponding methods for reproducing audio signals the amplification of the audio signals in one or more defined frequency ranges of the audio signals can be varied, on the one hand, and the amplification of the audio signals in a full effective frequency range can be varied on the other hand, the amplification in the defined frequency ranges being reduced if the amplification in the full effective frequency range is increased and the amplification of the audio signals in the defined frequency ranges being increased if the amplification of the audio signals in the full effective frequency range is decreased.
Amplifiers for use in audio reproduction systems have pitch control devices for varying the frequency response of the amplifiers in order to set the pitch level of the reproduced sound. Graphic equalizers are used as independent circuit components for fine tuning the frequency response in the amplifier connected thereto. Human ears suffer a natural loss of pitch sensitivity at relatively low and relatively high frequencies if the volume of the reproduced sound is low. Pitch control devices are used to compensate for this tendency of the human hearing system by raising or emphasizing high and low frequencies, so that the reproduced sound, as it is heard by the human ears, has an apparently flat frequency characteristic. With a pitch control, the set frequency characteristic of an audio signal can also be set manually by using a control knob or the like, but this is independent of variations in the volume. Therefore, the set frequency characteristic of an audio signal remains unchanged when the volume of the signal is increased or decreased.
In the design of audio amplifiers, the amplification is selected so that the reproduced sound is not distorted if the volume level is at the maximum setting in order to ensure powerful impressions on the listener if the audio signal is reproduced with a high output power and the reproduced sound is subject to a degree of distortion if an additional signal is applied to the amplifier raising the signal to a level above the permitted maximum level. Such an additional signal level is present, for example, when the volume level is raised while the low frequencies are emphasized by a pitch control device.
U.S. Pat. No. 5,138,665 discloses an audio reproduction system having an audio amplifier and a pitch control unit for controlling the pitch level by increasing or decreasing the amplification or by maintaining an increased or decreased amplification of a signal in a specific frequency range of a reproduced audio signal applied thereto, and a volume control unit for increasing or decreasing the amplification of the audio signal in the full effective frequency range of the amplifier. The pitch control and volume control units are coupled to one another in such a way that the amplification which would be increased in a specific frequency range by the pitch control unit can be controlled as a function of the degree to which the amplification has been increased by the volume control unit. The pitch control and volume control units can each comprise variable resistors whose contacts are connected to one another. If the knob of the volume control unit is turned to a higher level position, the amplification of the signal in the specific frequency range is reduced by the pitch control unit. If the knob of the volume control unit is turned to a lower control position, the amplification of the signal in the specific frequency range is increased by the pitch control unit.
Although the audio reproduction system described in U.S. Pat. No. 5,138,665 avoids the disadvantage of conventional audio reproduction arrangements that the reproduced sound is not distorted by the connection of the pitch control level to the volume control, it does not, however, allow for any external noise interference, such as occurs, for example, as a result of the operation of a motor vehicle engine and drive train i.e., the drive unit, when an audio signal reproduction arrangement is used in the motor vehicle. Because of such noise, the full sound pattern which is obtained in the stationary condition of the vehicle changes during travel.
German Offenlegungsschrift No. 33 22 055 discloses a circuit arrangement for automatically adapting the volume of a loudspeaker to a noise level prevailing at the location of the loudspeaker. For mobile radio receivers, the signal level has an adjustable functional network which generates a controlled variable for a volume controller as a function of the noise interference level so that a prescribed characteristic curve, which can be modified, at least in certain sections, as a function of the activation of the volume controller, is connected to modify useful signal level at the loudspeaker as a function of the noise interference level.
German Offenlegungsschrift No. 44 06 352 describes a car radio sound reproduction device having a noise interference-dependent volume control system with an audio frequency amplifier arrangement which can be controlled as a function of external noise interferences. For example, a signal which is obtained from the tachometer of a motor vehicle is supplied to a microprocessor which controls an equalizer arranged in the audio frequency signal path of the sound reproduction device. A prescribed, vehicle-specific characteristic diagram is stored in the microprocessor and is used to vary the audio frequency response and the volume setting as a function of the engine speed.
U.S. Pat. No. 4,553,257 describes an automatic volume control device by which the level of the useful input signal and the level of the ambient noise are detected. Both levels are subjected to modification in an arithmetic circuit. The amplification of the signal level of the sound signal takes place in accordance with a multiplication operation of the useful input signal and ambient noise level.